Many industrial methods for polymerizing olefins in hydrocarbons having 5 carbon atoms or less are known; typical examples of such methods include a method which comprises polymerizing ethylene in liquefied hydrocarbons having 4 carbon atoms (Philips' method), a method which comprises polymerizing propylene in liquefied propylene (Japanese Published Examined patent applications Nos. 17184/63, 24479/65, 39998/76, etc.), a method which comprises copolymerizing ethylene and propylene in liquefied propylene to obtain ethylene-propylene rubbers (Monte-Edison's method, Japanese Patent Publication Nos. 3291/1962 and 10491/1961, etc.), etc.
These methods all use hydrocarbons having a small number of carbon atoms as solvents in a polymerization vessel so that the polymerization conditions are generally required to maintain a high pressure of 5 kg/cm.sup.2 G or higher.
To recover the polymer from the polymer slurry by removing the solvent, a method which comprises reducing the pressure of the system from high pressure to thereby vaporize the solvent, the so-calleed flash method, has been adopted because the solvent is easily vaporizable (for example, see Japanese Published Examined patent application No. 24479/65).
In recovering the polymer, the polymer slurry obtained by polymerization is flashed as it is or after it is washed. Washing is one means for removing low molecular weight polymers, polymers having poor tacticity and catalyst residues, all of which are soluble in the solvent, and eliminating factors that have adverse influence upon physical properties (for example, stickiness of film, service stability, process stability, etc.) of final products of the polymer.
In this direct flash method for the polymer slurry, it is necessary to vaporize a large quantity of solvent, compress the vaporized solvent and if desired, purify the solvent by means of distillation, etc., and the consumed energy contributes much to the energy required for the overall process.
For this reason, a method which comprises sedimenting and condensing the polymer in a polymerization vessel or in a washing vessel and discharging the condensed polymer slurry has also been devised in order to reduce the amount of the vaporized solvent. To discharge the polymer in a slurry state, however, means that the quantity of the solvent discharged with the polymer is still large.
Paying attention to the foregoing problems and as a result of extensive investigations, the present inventors have found, as a method for greatly reducing the energy consumption as compared with the conventional methods, a method for separating and recovering a polymer from a polymer slurry which comprises filtering a polymer slurry under high pressure while maintaining the solvent in liquid phase, whereby the polymer is continuously obtained as a wet cake, and thus have accomplished the present invention.